but then he went back to afghanistan. >> i spent a lot of time with route clearance units looking for i.u.d.s. my vehicle never got hit. i was very, very lucky. when you run up there and pull open the door and see your injured buddies, you never forget that sight. >> this time back home, he began to explore the science of brain injury. >> what happens when the brain gets hit by a blast wave and slams up against the inside of the skull? >> with colleagues at new york university, parker built new tools to discover how a blast affects the brain. it could be years before this leads to new treatments, but parker hopes eventually will have a broad impact. >> a concussion a football player suffers or a car accident and your head snaps forward or shaken baby syndrome. these are all examples of head injuries that can cause a traumatic brain injury. >> today his lab is humming with young graduate students. about half a dozen are veterans. >> we're very focused on this because these are our buddies. these are our guys. this could have been us. it might still be us. so it brings a certain level o